1. Field of the Invention
The present invention relates to a novel technique for effecting coagulation/flocculation of aggregate-forming suspensions, and, more especially, to improved coagulation/flocculation technique characterized by enhanced cohesion among the individual flocs emanating from an aggregate suspension.
2. Description of the Prior Art
Coagulation/flocculation processes are widely employed in numerous fields of industry where there is a need to separate media comprising phases requiring separation, notably slurries. These processes are characterized by two principal steps or stages. The first step is actual flocculation, entailing aggregation of colloidal suspensions by, firstly, neutralizing the effect of the double layer of electrical charges (between the electrically-charged surface and a compensatory charge distributed throughout the mass of the solution) and, secondly, adding a flocculating or coagulating agent thereto. Such agent, which is typically an electrolyte or a polymer, such as polyacrylamide or polyaluminum chloride, initiates the formation of bridges that promote coalescence, i.e., flocculation. The particles thus obtained are then subjected to coagulation, via collisions between and among said particles. Generally, a collision is generated by motion of a mechanical origin, this being referred to as orthokinetic coagulation.
The solution containing the flocs thus obtained is then filtered, generally by filtration on a support or carrier. During this operation, numerous problems can arise, two of which predominate. The first of these is due to an absence of cohesion of the flocs which, as a result of attrition, disintegrate or break up into so-called microflocs, or fines, during their passage or transfer through valves, pumps, etc., and, as such, are able to destroy the equilibrium of the filter cake, be transferred into the filtrate, etc. The second problem is the presence of coagulant in the filtrate, this frequently being the result of using an excess of coagulant intended to overcome the difficulty indicated above.
"The behaviour of aggregates in stirred vessels," Trans. I. Chem., vol. 56, pp 9-18, Tomi and Bagster (1978), describes the relation between the size of the flocs and a cycle consisting of fast or rapid stirring at 500 rpm and slow stirring at 250 rpm, thereby effecting a distinct reduction in floc size as a function of the duration of stirring. "The role of rapid mixing time on a flocculation process," Wat. Sci. Tech., vol. 17, Amsterdam, pp 1091-1101 (Francois and van Haute) indicates that rapid stirring is necessary in the initial stage of coagulation, in order to obtain flocs having a high degree of cohesion. Such rapid stirring, with shear stresses (velocity gradient) of about 330 s.sup.-1 should nevertheless be carried out over a period of less than 150 seconds in order not to adversely affect floc growth and stability, and should satisfy several secondary criteria. Rapid stirring for a period of 30 seconds is described as being the optimum.